This invention relates to synthetic iron oxides. More particularly, the invention relates to synthetic red iron oxide prepared from soluble iron salts. More particularly, the invention relates to a method of improving the flow characteristics and dispersibility, such as in paint, of synthetic red iron oxide prepared from soluble iron salts.
Iron oxide, either natural or synthetic, is a well-known pigment. Among the synthetic oxides are black, yellows, browns, tans, and reds. The black and brown oxides contain varying ratios of ferrous and ferric oxides. Yellow iron oxides are the monohydrate of ferric oxide (Fe.sub.2 O.sub.3.H.sub.2 O). Tan oxides are a new class of iron oxide pigments, in which iron oxide is reacted with other metallic oxides to form a series of chemical compounds known as ferrites. For example, zinc ferrite theoretically contains 33.7% zinc oxide, but in practice varies from about 32% to about 34%, with the remainder being ferric oxide. Similarly, magnesium ferrite contains about 20% magnesium oxide. Synthetic red oxides can be made by 4 different methods. "Ferrite reds" are made by dehydrating the yellow oxide, thus converting the ferric oxide monohydrate to Fe.sub.2 O.sub.3. Another method is to precipitate black or brown oxide and then calcine at red heat to the ferric oxide state. The third method gives what is often referred to as "precipitated reds", made by direct precipitation from a solution of ferrous salts. The fourth method makes roasted "copperas reds". Copperas (FeSO.sub.4.7H.sub.2 O) is purified to remove other metallic salts and is then dehydrated to the monohydrate (FeSO.sub.4.H.sub.2 O). The monohydrate is calcined (roasted) to give red alpha Fe.sub.2 O.sub.3 (copperas red). The present invention is primarily concerned with improving the flow characteristics and dispersibility of roasted copperas reds, but is applicable to all synthetic iron oxides. "Copperas reds" are chosen as the preferred embodiment.
Prior art inventions in the field of iron oxide technology are exemplified by U.S. Pat. Nos. 2,184,738, 2,394,579, 2,416,138, 2,620,261, 2,980,617, and 3,009,821.
As mentioned above, copperas reds are typically prepared by calcining copperas. The crude product is then milled to give material of proper particle size, for good dispersibility in a medium, such as paint. A typical problem found in processing copperas reds, and other synthetic oxides, has been a low through-put of product through the mills, due to frequent plugging and jamming of the mills. The energy input required per pound of milled product has been high, resulting in a high cost product. Contamination of the ultimate product, from plugs breaking loose during milling, has been a common complaint.